1. Field of the Invention
The invention relates to a pipetting apparatus.
2. Description of the Related Art
A pipette of the mentioned kind is known from DE 25 49 477. In this, the closed work volume is formed in a cylinder and adjustable by moving an inserted piston. A linkage which is actuable by a button which can be thrown in works as an adjustment device. The work volume communicates with the hollow space of a detachable pipette tip so that by actuating the button liquid can be ejected or taken in by a lower tip opening. However, this piston travel pipette has the disadvantage that liberated contaminated vapors or aerosols of the probe liquid can reach into the upper cylinder space and can be delivered again out of control during the following steps of delivery together with probe material newly taken in. Subsequently, an encroachment of the following probes by contamination is possible even when the pipette tip is changed.
For overcoming these disadvantages, it already was proposed to arrange an exchangeable filter within the connecting region of pipette tip and pipetting apparatus. However, this solution uses an additional component, the filter, and is still unperfect with regard to the shielding of the work volume. Especially, it can be difficult to determine the right moment for exchanging the filter when reaching its maximum absorption capacity. An absolute contamination protection from the upper dose range is impossible.
Another suggestion for a pipetting appratus being shielded against contamination is known from the EP-A-0 077 180. Accordingly, the pipette tip is sealed by an annular sealing at the pipetting apparatus and has a capillary with a piston rod slidable in place. A piston with a seal element is attached removably to the piston rod, whereby the total length of this seal element must be at least twice the cylinder cross section. Prior to each sampling, a new pipette tip has to be mounted including piston rod to connect with the piston. Thus, several disposable parts are to be handled at each exchange of the dosing elements. The outer tip contour as well as the control unit of the piston are to be adapted carefully.
From U.S. Pat. No. 4,210,026, an apparatus for blood test sampling is known. Into this is insertable a one-ended sealed flexible envelope between a rigid and a hinged member. A rolling wheel is movable along the hinged member to compress the envelope progressively. When the wheel is retracted, the envelope can expand again, whereby the blood sample is drawn in. In this apparatus, the envelope is held safely only when it is compressed about its total length by the hinged member. Otherwise, it can float between the hinged and the rigid member, impeding the uptake of the sample. If the free space between the two members is opened relatively wide, the envelope may drop out. From the outside, it is difficult to observe whether it is held securely in the apparatus. Eventually, the sample volume drawn in is influenced by a certain variability of the arrangement of the envelope to the apparatus. Therefore, exact metering is impossible with the blood test sampling apparatus.